This document pertains in general to circuits that equalize non-ideal communications channels, and in particular to circuits that perform an equalization process to reduce inter-symbol interference when detecting transmitted symbols.
Integrated circuits typically communicate with one another and with other devices using conductive transmission lines. The conductive transmission lines may take the form of traces on a printed wiring board, cables, or the like. Integrated circuits typically include interface circuits that include drivers and receivers coupled to the conductive transmission lines. For example, an interface circuit may have a signal driver to drive electrical signals on one transmission line, and a signal receiver to receive different electrical signals from a second transmission line. Also for example, an interface circuit may have both a signal driver and a signal receiver coupled to the same transmission line for bi-directional communication using a single transmission line.
Interface circuits transmit digital bits, or xe2x80x9csymbols,xe2x80x9d on conductive transmission lines. A symbol may represent one or more digital bits of information. As the speed of communication increases, the symbols are transmitted faster, and the time distance between adjacent symbols becomes smaller. Signal drivers transmit symbols on conductive transmission lines, and signal receivers receive symbols on the conductive transmission lines.
An xe2x80x9cidealxe2x80x9d transmission line is a transmission line that conducts an electrical signal from one end to the other without distortion. In practice, perfectly ideal transmission lines do not exist. As a result, signals that are driven onto one end of conductive transmission lines emerge with varying amounts of distortion at the other end of the transmission line. As the distortion increases, and the communication speed increases, the distortion from one symbol may cause an adjacent symbol to be received incorrectly. This phenomenon is referred to as inter-symbol interference (ISI).
To partially alleviate the effects of ISI, a feedback control technique known as decision feedback equalization may be used at the signal receiver. One implementation of this technique converts a received transmission line analog signal into a digital received signal using a very fast analog-to-digital converter. A digital feedback signal is subtracted from the digital received signal to compensate for the distortion caused by the non-ideal transmission line. The digital feedback signal is typically created by an xe2x80x9cequalizerxe2x80x9d that models the transmission line, and predicts the correct digital feedback signal to xe2x80x9cequalizexe2x80x9d the distortion, and reduce ISI.
The feedback control technique just described can consume a significant amount of circuit resources and power. For example, very fast analog-to-digital converters can consume significant space and power, as can the circuits typically used to build equalizers.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternate methods and apparatus for reducing inter-symbol interference.